This invention relates to mirrors having the capability of being defrosted, defogged, or deiced and, more particularly, to a defrosting mirror especially adapted for outdoor use such as on the exterior of a vehicle or the like.
Defrosting mirrors and mirrors adapted for use in conditions wherein ice, frost, or moisture must be removed to provide clear, reflected images thereon have typically utilized a heat source applied to or secured near or adjacent the rear of a mirror element. In certain prior structures, resistive heating elements such as resistance wires or the like have been secured adjacent and generally parallel to the back of the mirror element and heated by the passage of electricity therethrough. The air surrounding and warmed by the elements is directed by moving air currents against the mirror element.
In other applications, resistance wire elements have been secured directly to the back of a mirror element in loop or sinuous patterns or secured between protective sheet materials which in turn are secured to the back of the mirror element. Such protective sheet materials such as asbestos have been proposed.
With the above type of heated mirrors, several drawbacks have been encountered. First, heating of the viewing surface of such mirrors was often nonuniform resulting in areas of the mirror which were deiced faster than other areas which were not deiced at all. Further, certain areas were heated to too high a temperature causing damage to the mirror elements either from breakage of the glass or other mirror substrate or degradation of the reflective coating on the mirror. Also, such mirrors, especially when used in exterior applications were subject to degradation from moisture, such as rain and/or humidity as well as the corrosive effects of salt and other chemicals present in the atmosphere especially in areas close to an ocean. This latter problem was aggravated where insulation coverings for the resistance wires were used which tended to absorb the moisture and/or salt spray. Moreover, the cost of the above type of mirrors tended to be very high even though the results obtained were often not satisfactory.
Another type of prior known heated mirror structure is that utilizing a chrome, nickel-chrome, or other thin metallic layer applied to one surface of a glass, plastic, or other mirror substrate. In certain instances, the metallic layer served both as a light-reflective layer as well as a heat conductive, resistance layer. In other applications, two layers of metallic material were used, one for light reflection and the other for heat generation. However, these types of mirrors also suffered from drawbacks during use.
For instance, it was often difficult to obtain adequate, long-term securement or adhesion to the mirrors of metallic bus bars used to convey electricity to the metallic layer. Also, because a certain thickness of metallic material was necessary to provide adequate light-reflective qualities, it was often impossible to properly control the metallic layer thickness to provide the correct electrical resistance and thus, the correct heat generation qualities. Conversely, control of the metallic layer thickness for proper heat generation qualities often produced inferior light-reflecting qualities. Further, there was often large electrical resistance encountered between the bus bar and the metallic layer resulting in problems with electrical conduction. Moreover, in those applications in which a single layer served both to reflect light and generate heat, scratches in the metallic layer would often effectively prevent electrical conduction and thus heat generation to some or all of the mirror surface even though light reflection would not necessarily be severely impaired.
In addition, other problems were encountered especially when heated mirrors were used on vehicles such as cars or trucks and subjected to snow, sleet, rain, and other severe weather conditions. Often, the heating mechanisms were so delicate that they would function improperly in anything less than optimum weather conditions.
Therefore, a substantial need has been evidenced for a defrosting or heated mirror which is durable, inexpensive to manufacture, provides high quality optical reflective qualities, and yet also provides efficient, reliable defrosting qualities even in severe environmental or atmospheric conditions. This need has been especially great for exterior mirrors used on cars and trucks, which, of necessity, must function properly in severe and widely varying conditions.